The present invention relates to a thermal relief valve and to a method for making the thermal relief valve.
Relief valves are part of virtually all piping arrangements that contain liquid having a differential of either pressure or temperature. Typically, pressure relief is required in a piping arrangement that includes a positive displacement pump, such as a rotary pump or a reciprocating pump, in order to reduce overpressurization at the pump and of the system. Thermal relief may also be required in a piping arrangement that conveys a liquid that is susceptible to transient temperature excursions.
One type of system vulnerable to a transient temperature excursion is a piping arrangement system that recirculates water or other liquid with a pump. If the volume of liquid recirculated is low, and the liquid is circulated at a high rate, it is possible for the liquid temperature within the piping arrangement to increase while the liquid is recirculated due to the heat generated by the pumping action.
To prevent the temperature of the liquid from exceeding a temperature that would cause damage to components in the system, the piping arrangement is typically provided with a type of a thermal relief valve. The thermal relief valve reduces temperature and permits make-up of cooler liquid under pressure into the piping arrangement system.
Usual operation against a closed discharge of the pump is both infrequent and of a very short duration. To modulate damaging consequences of this operation, an automatic unloader valve with an external return connection is preferably used. Liquid redirected by the unloader valve is typically piped back to the source of supply.
Types of thermal relief valves include a "squeeze-push valve" and a "diaphragm valve." The squeeze-push valve includes a synthetic rubber bag that surrounds a stainless steel piston. The synthetic rubber bag is enclosed within a main body having a cup and a cap over the cup. The stainless steel piston protrudes from the cap. Within the cup is a wax. The wax melts at a predictable predetermined temperature. When the valve is heated, the wax melts, pressurizing the cup and the cap. As a consequence, the rubber bag collapses and forces the piston out of the cap by both a lateral squeeze and a vertical push.
The diaphragm valve is one where a synthetic rubber diaphragm separates the wax within the cup from a piston within a piston guide. The piston guide is tapered with respect to diameter. Above the rubber diaphragm, within the piston guide, is an oversized plug. An increase in temperature causes the wax to expand, applying pressure to the rubber plug. The rubber plug is then forced into a reduced diameter in the tapered piston guide.
The cup of a thermal relief valve is typically hollow and encloses a wax. The wax melts at a predictable predetermined temperature. When the cup is heated, the wax within the cup melts and expands. The melted wax pushes against the piston, moving the piston. Movement of the piston forces the body to unseat from the o-ring. Once the piston is moved, hot liquid is vented from the system. The venting continues until the liquid is cooled due to a concurrent cool liquid make-up to the system.